Which units would you replace in the Sylvan castle?

Indeed. And as we know from the Elder Edda, Valkyries no longer fight; instead, they bestow upon chosen heroes (read: their beloveds) a kind of magical protection from various harms in battle and protect them from witches. In general, they ensured victory in battle and collected the souls of fallen warriors. However, one "demoted" Valkyrie, punished for disobedience, was a very strong warrior, although she did not participate in battles.